The present invention relates to a communication system implementing time division multiple access in which a number of terminal stations communicate with a central node and wherein information is sent in the form of bursts from the terminal stations towards the central node. The invention also relates to a method of communicating information, from a number of terminal stations to a central node, using time division multiple access.
The invention still further relates to a method of rearranging the transmission positions for bursts of information sent from a number of terminal stations in a point-to-multipoint system implementing time division multiple access depending on the transmission demand of the terminal stations.
In point-to-multipoint radio communication networks information is communicated between a central node (a radio node) and a number of terminal stations, also called radio terminals. Each radio terminal in turn continuously communicates with a user station arranged next to the terminal and the radio node continuously communicates with a transmission network. For communication from the radio node to the radio terminals, i.e. for downlink communication, the transmission is continuous and all terminals have got access to all the information, i.e. information is broadcasted. However, for communication from the terminals to the radio node, i.e. uplink communication, time division multiple access is implemented which means that the radio terminals communicating with the radio node share a common communication resource within which each radio terminal is assigned a time period during a frame, (the communication channel is divided into a number of frames), during which it is allowed to send a burst of information.
Thus information is sent in both directions between the transmission network and the users. During steady state operation, i.e. during normal operation, the radio terminals transmit regularly with a constant time interval, a frame, in which each terminal is assigned a given capacity and a given starting position so that the total capacity can be used as efficiently as possible which means that the bursts from the respective terminals are packed as densely as possible within the frame. However, the amount of data may vary from terminal to terminal which means that the bursts which are sent do not have to have a constant length in relation to each other. During the time period it takes to send a frame, all the active radio terminals must be allowed to send information at their predetermined time positions. The radio node comprises scheduling means which is used to determine these time positions.
In a point-to-multipoint access network it often occurs that the transmission demand, e.g. the amount of information that needs to be sent from terminals varies, or terminals may be disconnected as well as new terminals may be connected etc. It is then important that such changes can be handled in such a manner that the transmissions channels still can be used in an efficient way. This is done through rearranging the frame or by defragmentation of the frame. In order to provide for defragmentation, the time positions for sending bursts from the respective terminals have to be changed for example in order to enable the sending of information from a new connected terminal, i.e. that a time interval has to be made available to such terminal. The displacement of the transmission positions for the terminals must be handled in such a way that the new terminal is allowed to transmit information or that a terminal needing to send more information is enabled to do so etc. In a system not permitting reconfiguration of the transmission positions of the bursts from the respective terminals, buffering means provided in the radio node as well as in the radio terminal would only have to store the information corresponding to the number of information bits arriving during a frame. This however means, for example, that if a terminal is disconnected, there is a risk that the capacity of the channel might not be fully used.
However, a number of systems are known which permit the displacement of the transmission positions. FIG. 1A schematically illustrates bursts from a terminal, here denoted terminal T, which are sent in a first, early, position within the frame and the sending positions for which are changed to a later position within the frame. It should be noted that information or data continuously arrives to the terminal from the user station as well as data continuously is sent out from the radio node to the transmission network. Furthermore each burst contains the same amount of information and the information is buffered in buffering means BT in terminal T as well as in the radio node. As can be seen from FIG. 1A in frames Nxe2x88x921 and N, bursts BNxe2x88x921, BN are sent at a first position within the respective frame whereas in frames N+1 and N+2 burst BN+1, BN+2 are sent at a new, later position within the frames. This means that between frame N and frame N+1 the buffering means of the radio terminal has to store more information and the radio node has to send out prestored information.
This means that the total filling degree in the buffering means (terminal and node) is constant within the system but that data has been redistributed from the node to the terminal buffering means. This is graphically illustrated in FIGS. 1B and 1C respectively. In FIG. 1B the amount of data in the buffering means of the radio terminal is illustrated as it changes with time, i.e. from the sending of frames Nxe2x88x921 to N+2. Data in the buffering means of the radio terminal DBT during steady state operation maximally reaches a given value and as a burst is sent during frame Nxe2x88x921, the data in the buffering means of the terminal station is sent out. Thereafter, during the rest of the frame, data is collected until the sending of a further burst in frame N. However, since a displacement from an early position within the frame to a later position within the frame takes place during the time that passes between the sending of frame N to frame N+1, the data in the buffering means of the radio terminal is substantially doubled until a burst is sent out at the later position as illustrated with a burst BN+1 in frame N+1. Then, again, a steady state operation is reinstated which however requires a larger buffer in the radio terminal. In FIG. 1C the corresponding situation for the buffering means of the radio node is illustrated and the amount of data in the node buffer DBN is illustrated as a function of time. As can be seen from FIG. 1C, the opposite situation occurs in the node buffering means and it will be realized that the sum of the data in the buffers in the radio node and in the terminal respectively, illustrated through X and Y respectively in FIGS. 1B and 1C, before and after the displacement, is the same, which means that the delay in the point-to-multipoint communication system will be the same but data has been redistributed to the buffering means of the radio terminal from the radio node. Thus, the same amount of data is sent every time but the sending is set out once which means that a larger delay is produced and large buffering means are required.
In order to allow the defragmentation of a channel it must also be possible to displace bursts from a later position to an earlier position within a frame. This is described with reference to FIGS. 2A-2C corresponding to FIGS. 1A-1C but with the difference that the change produced in frame N+1 is that bursts are sent at an earlier position. As can be seen from FIG. 2B it is a prerequisite by that the terminal has prestored data if it is to be able to send two bursts of the same size within a short period of time It is supposed that the amount of data is equal in each burst. There must also be free capacity in the buffering means of the node in order to be able to handle a fast filling up of the buffering means since two bursts of substantially the same size are received within a short period of time. Thus, the amount of information that is to be accumulated in the buffering means is given by the rate at which data is transmitted, the frame frequency and the maximum displacement of a burst within the frame. In a system admitting free displacement of a burst, i.e, from an earlier to a later position within a frame and vice versa, the buffering capacity has to be increased in proportion thereto, i.e. in proportion to the maximum displacement. This means that if a displacement of bursts throughout the frame is to be permitted, from one end position to another, the capacity of the buffering means must be doubled having as a consequence that also the delay is doubled. This is extremely disadvantageous since the delay in a transmission network should be as small as possible and a delay may lead to the production of echoes etc. for example in speech connections.
U.S. Pat. No. 4,843,606 shows a local area communication system comprising token rings which are interconnected by a time division multiplex unit via synchronous bandwidth managers issuing priority tokens for synchronous frames at regular intervals. Each synchronous bandwidth manager can be seen as a terminal unit which, depending on the demand, is assigned different bandwidth on the time multiplexed network. The assigned starting time positions can be rearranged in order to use the network efficiently. The starting position for transmission from a particular terminal changes successively over a frame during which two time slots are occupied which means that the same information is sent in the old as well as in tone new time slot. The object is to ensure a secure rearrangement and the same information is sent in two time positions.
What is needed is therefore a communication system using time division multiple access (TDMA), particularly a point-to-multipoint transport or access network, which can handle changes when the amount of information to be sent from terminals changes, when terminals are connected, disconnected etc. so that the common transmission channel is used in an efficient manner. Particularly a system and a method are needed through which it is possible to handle such changes without affecting the transmitted information and without loosing information. Particularly a system and a method are needed through which the delay can be kept as low as possible, and even more particularly be minimized. Particularly a system and a method respectively is needed through which the capacity in frames on the uplink from a number of terminals having a varying need of bandwidth towards a central node can be used in an efficient manner. Further yet a system and a method are needed through which the buffering capacity of terminal stations as well as of a central node can be kept as low as possible and independent of occurring changes. Particularly a system and a method are needed able to handle the situation when the bursts from different terminals have different lengths.
Therefore a TDMA system is provided which comprises a number of terminal stations each of which communicates with a number of user stations and with a central node. Information to the central node from a number of terminal stations is transmitted in frames as bursts of variable lengths and separate frequencies are used for communication to and from the central node respectively. Each terminal station comprises terminal station buffering means and the central node comprises node buffering means for buffering of information to be transmitted. In the node scheduling means are provided for controlling the transmission of information in bursts from the terminal stations towards the central node. The central node comprises means for collecting information about actual transmission demands from each terminal station and about connection/disconnection of terminals. The scheduling means comprises control means for controlling the use of the frame structure for each terminal in agreement with the aforementioned information through changing the sending of bursts within the frame structure without requiring the buffering means in the central node and in the terminal stations to exceed a given size which is given by the size required for steady state transmission so that the transmission delay in the system is minimized.
The node comprises means for introducing a transitional frame during which the terminal station performs a change in its transmission of information. In a particular embodiment the change in transmission of information from a terminal station comprises a change to sending a burst starting at a new position which is later in the frame than an old position at which a burst from the same terminal was sent in the preceding frame(s). Alternatively the change in transmission of information may comprise sending of a burst from a particular terminal station at an earlier position within a frame in relation to the position at which the burst from the corresponding terminal stations was sent in the preceding frame(s). In order to provide the means for maintaining information about the actual transmission demands of the terminal stations, signaling is used between the terminal stations and the node.
Advantageously the scheduling means decides which changes are to be undertaken depending on the information about transmission demand etc. that the scheduling means receives. The scheduling means also transmits information thereon to the terminals and synchronizes the changes in the terminal stations and the node.
Advantageously the sizes of the terminal station buffering means and the node buffering means respectively are given by the length of the frames and by the information transmission rate independently of the changes relating to transmission positions of bursts within a frame and the length of the bursts.
If the transmission position of a burst is displaced from an earlier to a later position within a frame, two bursts are sent at different positions during the transitional frame of which bursts one is a transitional burst. According to one embodiment a burst is sent at the earlier, or the old, position and a transitional burst is sent at the new position, which transitional burst contains the data collected in the buffering means of the terminal station since the sending of the preceding burst, i.e. the burst sent at the earlier position. In an alternative embodiment a transitional burst is sent at the earlier, old, position and in addition thereto a burst is sent at the new position within the transitional frame. Advantageously the number of bits in a transitional burst is the product of the rate at which information/data is received in the terminal station, the length of the frame and the proportion of the length of the frame that the transmission position of the burst is to be displaced. Thus, the length (size) of the transitional burst can be precalclated. In the subsequent frame, i.e. the frame following on the transitional frame, only a burst is sent in the new transmission position.
In another case the transmission position of a burst is displaced from a later to an earlier position within the frames. Then, during the transitional frame, only a transitional burst is sent which comprises at least part of the information collected since the preceding sending of a burst in the preceding frame, i.e. the frame preceding the transitional frame. According to one embodiment a transitional burst is sent substantially in the new position, as assigned by the scheduling means, during the transitional frame. A burst contains the data collected in the terminal station buffering means since the preceding transmission of a burst in the frame preceding the transitional frame.
In an alternative embodiment a transitional burst is sent during the transitional frame substantially at the old position which burst contains part of the information/data collected in the terminal station buffering means. In a particular embodiment the transitional burst is smaller than the burst sent in the old position and it ends at the end position at which a new burst of substantially the same size as a burst sent at the old position, should hive ended.
According to the invention the size of the transient burst can be calculated in advance; this is thus known to node and terminal and no signaling or handshaking is needed; i.e. no communication is needed as to the number of bits etc; all what is needed is information about whether to displace in one or the other direction and the amount of displacement.
Advantageously the number of data/information bits in the transitional burst, when displaced from a later to an earlier position within the frames, comprises the product of the data rate at which data is received in the terminal, the length of the frame and 1- (the proportion of the length of a whole frame that the burst is to be displaced). If the transmission position of a burst is to be displaced from an earlier to a later position within the frame by a distance which is smaller than the length of the old (ordinary) burst, the length of an old (ordinary) burst is extended so as to contain the bits received during the sending of a burst having the length of an old burst.
Correspondingly, when a burst is to be sent at an earlier position within the frame and the displacement is smaller then the burst itself, a burst which is smaller than an old (ordinary) burst is sent out during the transitional frame.
A method of communicating information from a number of terminal stations continuously receiving information from user stations to a central node is also provided. The central node continuously delivers information to a transmission network. The information is communicated from the terminal stations to the central node on a transmission channel divided into a number of frames using time division multiple access and the terminal stations send information in bursts at given first positions within the frames. The method comprises the steps of collecting information in the central node about the transmission demands of the terminal stations and about connection/disconnection of terminal stations, determining in the node, with the use of said information, whether the transmission positions of bursts from a terminal station needs to be displaced to an earlier or to a later position within the frames to meet the transmission demands to optimize the usage of the channel capacity, and providing the terminal station with information about a new transmission position. Furthermore it comprises the steps of introducing a transitional frame during which the change in transmission position is handled, which comprises the sending of at least a transitional burst without requiring enchanced buffering capacity neither in the node nor in the terminal station. In the subsequent frame, i.e. the frame following on the transitional frame, a burst is sent at the new position within the frame. In one case the transmission position of a burst within the frame is changed to a later position and the method then comprises the step of sending two bursts during the transitional frame, of which one burst is sent at the old position and the other at the new position and wherein one of the burst comprises a transitional burst of a shorter duration. According to one embodiment the method comprises the sending of a first burst at the old position, collecting received information bits in buffering means in the terminal station and sending a transitional burst at the new position comprising the information bits collected in the buffering means of the terminal station since the sending of the preceding burst from the same terminal within the transitional frame.
According to a preferred implementation the size of the transitional burst is calculated in the node wherein the size is determined using among others information about the length and direction of displacement.
In an alternative embodiment the method comprises the steps of sending a transitional burst at the old position within the frame, which burst comprises part of the information bits collected in buffering means in the terminal station and sending a further burst at the new position within the transitional frame which burst comprises the information bits collected in the terminal station buffering means.
The method also relates to changing the transmission position of the burst from a terminal station to an earlier position within the frame which includes the step of, in the transitional frame, only sending a transitional burst. Advantageously the transitional burst sent at the new position ends at the end position of the new position.
In an alternative embodiment the method comprises the step of sending a transitional burst at the old position within the frame which burst comprises only part of the information collected in the terminal buffering means since the preceding transmission of a burst from the same terminal in the preceding frame.
The invention also provides a method of rearranging the transmission positions for bursts of information sent from a number of terminal stations in a point-to-multipoint system using time division multiple access depending on the transmission demand of the terminal stations. The method comprises the steps of, collecting information in a central node to which information is provided about the transmission demands of the terminal stations (including connection/disconnection of terminal stations), establishing with use of said information whether a burst of a particular station needs to be sent at an earlier or at a later position within the frames, and introducing a transitional frame during which information is sent in at least a transitional burst so that in a subsequent frame the terminal station can send its burst at the new position and without requiring any additional buffering capacity neither in the terminal station nor in the central node for performing the rearrangement of transmission position. Particularly the method comprises the step of changing the sending position from an earlier to a later position within a frame through sending one burst at the old position and sending another burst at the new position within the transitional frame. The method also includes the step of changing sending position for a burst from a later to an earlier position within the frames, in which case only a transitional burst is sent during the transitional frame. The invention particularly relates to the use of the system as referred to above for speech and/or data communication in transmission channels on for example fiber links, coaxial cables or copper cables. Particularly the system is used for microwave communication point-to-multipoint systems but also for any other point-to-multipoint communication system.